1. Field of the Invention
The present invention relates in general to combustion chambers for spark ignition internal combustion engines and more particularly to improvements in the shape of a combustion chamber.
2. Description of the Prior Art
A combustion chamber for a spark ignition internal combustion engine is shaped as for example shown in FIG. 1.
In FIG. 1, designated by the reference numeral 1 is a cylinder head, by 2 a cylinder block and by 3 a piston. Between these a combustion chamber 4 is defined. The cylinder head 1 is provided with an intake valve 5, a spark plug 6, etc.
In such combustion chamber 4, when the ignition spark by means of the spark plug 6 occurs, flame spreads from the zone around the spark plug 6 in all directions. In this instance, since the top of the piston 3 comes close to the spark plug 6 at the time of the ignition spark, a considerable part of the flame front collides with the flat top of the piston 3 and is checked thereby from propagating further.
For this sake, the propagation of the initial flame front is not smooth nor stable, causing the possibility that the flame cannot sweep throughout the combustion chamber 4 within a short interval or duration. This is particularly true when the engine is operated under some specific conditions.
With a view to protecting the initial flame front against such check, there has been proposed, as shown in FIG. 2, a piston 7 that has wedge-shaped depressions 7A and 7B formed in the top of the piston so that the rate of the initial combustion zone to the entire combustion chamber (hereinafter will be referred to as the rated initial combustion zone) is increased.
With this piston 7, it becomes possible to lengthen the interval during which the flame front moves from the spark plug 6 to the top of the piston 7 so as to permit the flame to spread to almost all areas or zones of the combustion chamber 8 during the lengthened interval.
When, however, a swirl or squish is produced so as to permit the charged gas to flow within the combustion chamber 8 in order to obtain efficient combustion of a lean mixture or a mixture under a high EGR (exhaust gas recirculation) rate, such shape of the combustion chamber 8 provides even worse conditions for short combustion duration since it tends to supress the desired gas flow within the combustion chamber.